The present invention relates to a method for controlling a braking force of a vehicle. The invention further relates to a method for preventing tip-over of a work vehicle during traveling. The present invention also relates to a vehicle braking system and a work vehicle comprising the braking system.
The term “work vehicle” comprises different types of material handling vehicles like construction machines, such as a wheel loader, an articulated hauler, a backhoe loader, a motor grader and an excavator. Further terms frequently used for work vehicles are “earth-moving machinery” and “off-road work machines”. The invention will be described below in a case in which it is applied in a wheel loader. This is to be regarded only as an example of a preferred application.
The work vehicles are for example utilized for construction and excavation work. A wheel loader may be used to transport heavy loads from one location to another, often encountering a series of turns and varying grade slopes on the route between two or more locations. If the load arm unit is in a raised position during traveling and the vehicle has a high speed, a hard braking action may cause an unstable condition, sometimes causing the rear wheels of the wheel loader to lift from the ground and providing discomfort to the operator. In extreme conditions, the wheel loader can tip over in the traveling direction.
Further, there are requirements on a minimum available retardation speed for a fully loaded vehicle when traveling on a road. The requirements on a minimum available retardation speed gives rise to very powerful vehicle braking systems. However, such powerful vehicle braking systems may increase the above mentioned problems when traveling with the load arm unit in a raised position.
Further, today many work vehicles are used by unexperienced operators. The above mentioned problems when traveling with the load arm unit in a raised position are further increased for unexperienced operators.
Accordingly, a method and a system is desired which operates reliably, protects the load, machine, and operator from a tip-over, provides operator comfort and acceptability, encourages operator confidence which may lead to more rapid maneuvering and greater productivity, and may be used in a timely and efficient manner.
U.S. Pat. No. 6,437,701 discloses a machine stabilization system for an articulated work machine. A plurality of operating parameters comprising the weight of the load and the implement position are detected and a stability value is determined based on the detected operating parameters. The stability value is compared to an alarm value. Should an instability condition be detected, an output signal is sent to an output device to alert the operator and/or affect the movement of the machine to prevent instability of the work machine. The output device may be a steering system or a braking system.
It is desirable to achieve a method for controlling a braking force of a vehicle in motion, which improves reliability and safety.
According to an aspect of the present invention, a method is provided for controlling a braking force of a vehicle, comprising the steps of detecting at least one operating parameter of the vehicle, detecting a position of an operator controlled braking element, determining a magnitude of a braking force on the basis of a detected magnitude of the operating parameter and the detected position of the operator controlled braking element and braking the vehicle according to the determined braking force. The braking method is preferably performed via a so-called brake-by-wire system.
According to a preferred embodiment the operating parameter comprises a force subjected to the vehicle. Such a force may for example arise from a vehicle load, such as a load (material) carried by an implement of a wheel loader, or a load (material) on a rear load carrier of an articulated hauler or a truck. In addition to the force from such vehicle loads, there is a force from the weight of the vehicle itself. Preferably, the total force subjected to a rear axle of the vehicle is determined/detected. The total force is preferably detected by means of a bearing sensor in the rear axle.
According to an alternative or complement to determining a force on the vehicle rear axle, a detected magnitude of a vehicle speed and/or a detected magnitude of a vertical position of an implement of the work vehicle may be used for controlling the braking force. Further, a detected value of a load of an implement of the work vehicle may be used for controlling the braking force.
According to a further preferred embodiment, the method comprises the step of selecting a braking force response from a stored set of responses on the basis of the detected magnitude of the operating parameter. Each stored response may comprise an individual function/algorithm and/or a look-up table.
According to a further preferred embodiment, the different stored braking force responses comprise different magnitudes of the braking force in response to a specific position of an operator controlled braking element. Preferably each individual position (or an interval of positions) in the whole range of positions of the operator controlled braking element corresponds to different magnitudes of the braking force.
According to a further preferred embodiment, the method comprises the step of calculating the braking force as a function of the detected magnitude of the operating parameter. Preferably, the braking force value has an inverted relation with regard to the detected magnitude of the operating parameter.
It is also desirable to achieve a method for preventing tip-over of a work vehicle with an implement for carrying a load, during traveling, which improves reliability and safety.
According to an aspect of the present invention, a method comprises the steps of detecting at least one operating parameter of the vehicle, detecting a position of an operator controlled braking element, determining a stability value based on the detected operating parameter and modifying a brake signal from the operator controlled braking element to a slower and/or decreased response if the stability value is above a predetermined alarm value. Thus, the alarm value defines a risk zone for vehicle tip-over.
Thus, the operator is not allowed to order a braking action which will cause tip-over of the machine. Thus, the braking force is reduced for work states, which are predefined based on the determined operating parameter (s).
Preferably, the stability value is calculated as a function of the operating parameter(s). The alarm value is preferably predetermined so that it will be reached when at least one of the operating parameters is above a predetermined value. Thus, the braking force is optimized for a certain work condition.
According to a preferred embodiment, the maximum braking force is unlimited when a maximum brake force is required from a vehicle operator. Thus, as a security measure, full braking capacity should be available if necessary. In other words, availability of the full braking capacity of the vehicle braking system takes priority/precedence over the tip-over braking force limitation.
Further preferred embodiments and advantages will be apparent from the following description and drawings.